Solid oxide fuel cells (SOFC's) are well known in the art and come in various designs. Typical configurations include a flat plate design and a tubular design, wherein an electrolyte layer is sandwiched between two electrodes. During operation, usually at a temperature from 500° C. to 1100° C., one electrode is in contact with oxygen or air and the other electrode is in contact with a fuel gas.
Several properties are required for the SOFC's, such as high conductivity, a large area of electrochemically active sites at the electrode/electrolyte interface, chemical and physical stability over a wide range of fuel atmospheres, and minimal microstructural changes with operating time, since such changes are often accompanied by deterioration of electrical performance.
Under typical operating conditions, a single cell produces less than 1 volt. To obtain high voltage and power from the SOFC's, it is therefore necessary to stack many cells together.
The most common manufacturing method for SOFC planar stacks comprises the manufacture of single cells. The cells are subsequently stacked together with interconnects, current collectors, contact layers and seals. After assembly the stacks are consolidated/sealed by heat treatment under a vertical load, to ensure sealing as well as electrical contact between the components. The mechanical/electrical coherence of the stack during operation is ensured by applying a constant vertical load (using, e.g., a yoke).
The cells are most commonly manufactured by the use of wet powder processing techniques which comprise tape-casting of the support component (typically the electrolyte or anode). The cell support component is normally made by tape casting of a powder suspension and the active layers (cathode, electrolyte and anode) are subsequently deposited onto the support by spray painting or screen printing, with intermediate sintering steps for the different layers.
Alternatively SOFC's are for instance manufactured by electrochemical vapour deposition (CVD) method or plasma spraying. However, said processes are very expensive, and thus there has been a desire to lower the fabrication costs.
Consequently, when a planar SOFC is designed, one emphasis is to minimize sealing and sealing surfaces because the seal requirements are very stringent. Suitable high-temperature sealants being used include cements, glasses and glass-ceramics. A sealant selected for planar SOFC's must have sufficient stability in oxidizing and reducing environments, chemical compatibility with cell-stack components, and proper sealing and insulating properties. Examples of glass and glass-ceramic sealants being developed for planar SOFC's are modified borosilicate and aluminosilicate glasses.
US-A-200400115503 discloses an electrochemical device assembly, comprising a porous electrically conductive support layer; a prefabricated electrochemical device layer; and a bonding layer between said support layer and said electrochemical device layer. Also disclosed is a method of fabricating said assembly, said method comprising: providing a porous electrically conductive support layer; providing a prefabricated electrochemical device layer; and bonding said support layer and said electrochemical device layer with a bonding layer.
U.S. Pat. No. 6,458,170 relates to a method for fabricating a bilayer structure consisting of a porous substrate with a bonded dense film, comprising the steps of: forming a homogeneous porous substrate layer and curing it to a predetermined green density, applying by means of an aerosol spray a uniform suspension of a film material in a volatile carrier onto the surface of said substrate to form a thin green film layer of a predetermined thickness, volatilizing said carrier, and firing the bilayer formed by said green film layer and said substrate layer to sinter thin green film layer and substrate, said substrate having a predetermined green density selected such that the total shrinkage of the fired green film layer and fired substrate layer is such that the film shrinkage is equal to or less than that of the fired substrate.
Y. Matus et al., “Metal-supported solid oxide fuel cell membranes for rapid thermal cycling”, Solid State Ionics, 176 (2005), 443-449, relates to SOFC membranes, in which zirconia-based electrolyte thin films are supported by a porous composite metal/ceramic current collector and are subjected to rapid thermal cycling between 200° C. and 800° C.
U.S. Pat. No. 6,843,960 discloses a method of making metal or metal alloy plates, comprising the steps of: obtaining a powder of a predefined composition, adding solvents, dispersants, a plasticizer and an organic binder to said powder to form a slip; forming said slip into a layer on a substrate; forming an additional layer directly on said layer and forming a plurality of additional layers directly on previous layers to provide a multiple layer graded stack in a defined order; heating said multiple layer graded stack to a predefined temperature for burning out said binder; and sintering said layer in a reducing atmosphere at a set temperature for a predetermined duration.
US-A-20030232230 relates to a SOFC repeat unit comprising a multilayer laminate, said multilayer laminate including a metallic air flow field; a metallic interconnect disposed on said metallic air flow field; a metallic fuel flow field disposed on said metallic interconnect; an anode disposed on said metallic fuel flow field, and an oxide electrolyte disposed on said anode. The obtained sintered repeating units are stacked in order to form a SOFC, and the stack is afterwards sintered and sealed.
WO 03/075382 teaches a solid oxide fuel cell of the type including repetitively stacked anode, electrolyte, cathode and interconnect layers including a plurality of gasket elements which space apart the interconnect, electrolyte, and gasket elements also defining integral manifolds for the fuel and oxidant inlet and exhaust flow streams.
GB-A-2400723 discloses an intermediate-temperature solid oxide fuel cell, comprising a ferritic stainless steel substrate including a coarsely porous support and a non-porous frame housing said porous support, a first electrode layer located in the non-porous frame and supported internally by the coarsely porous support, an electrolyte layer located over the first electrode layer, and a second electrode layer located over the electrolyte layer.
US-A-20020048699 relates to a SOFC, comprising a ferritic stainless steel substrate including a porous region and a non-porous region bounding the porous region; a ferritic stainless steel bi-polar plate located under one surface of the porous region of the substrate and being sealingly attached to the non-porous region of the substrate about the porous region thereof; a first electrode layer located over the other surface of the porous region of the substrate; an electrolyte layer located over the first electrode layer; and a second electrode layer located over the electrolyte layer.
WO 02/09116 discloses a SOFC of the type including repetitively stacked anode, electrolyte, cathode and interconnect layers including a plurality of gasket elements which space apart the interconnect and electrolyte elements and bound to the anode and cathode elements. The interconnect, electrolyte and gasket elements also define integral manifolds for the fuel and oxidant inlet and exhaust flow streams.
U.S. Pat. No. 6,248,468 describes a process for obtaining a fuel cell, including the steps of providing a pre-sintered nickel-zirconia fuel electrode, and an air electrode with a ceramic electrolyte disposed between the electrodes. The fuel electrode is sintered to provide an active solid oxide fuel cell.
U.S. Pat. No. 5,908,713 relates to a method of forming a fuel electrode on an electrolyte of a SOFC by a sintering process, the method comprising the provision of an underlayer to which an electrolyte is applied in the form of a slurry, which is then dried. An overlayer is thereafter applied to the underlayer and then dried. The dried underlayer and overlayer are then sintered to form the fuel electrode.
However, there are a number of disadvantages with the designs and manufacturing procedures of SOFC stacks hitherto known:    1. True monolithic planar stacks are not obtained since the mechanical integrity of the stacks requires a permanent mechanical load to maintain sealing and electrical contact during operation.    2. The manufacturing processes are complicated and comprise numerous sintering steps or expensive chemical or physical deposition techniques.    3. In the case of sintering of the electrodes, there are two drawbacks:            a. Due to the required sintering temperatures, performance limiting interface reactions are often observed between the electrodes and the electrolyte and/or interconnect;        b. During sintering it is not possible to maintain sufficient fine microstructures in the electrode and in the electrode/electrolyte interface due to excessive grain growth.        